This invention relates to fluid pulsation attenuators, that is, mufflers or acoustic rejection filters for reducing the fluctuations in the flow of fluids in conduits. More specifically, it relates to a valve for controlling the flow of air into and out of the air chamber adjacent one side of the diaphragm in a pulsation attenuator of the type used in the stock conduit feeding an aqueous slurry of stock fibers into the headbox of a papermaking machine. Still more specifically, the invention relates to a three-port, three-way valve wherein the rapid entry and rapid release of air, at start-up and shutdown, and the more leisurely controlling of the air pressure in a chamber on one side of the attenuator diaphragm to quickly achieve matching, and to continue the matching of this air pressure to the running-average pressure of the fluid on the other side, is achieved by movement of a sliding tube within the valve.
Prior valves used in these pulsation attenuators were two-port, two-way valves and were of two general types. In conjunction with either of these valve types, pressurized air was continuously introduced at a selected rate, approximately scaled to chamber volume, into the air chamber adjacent the attenuator diaphragm via a separate air input path. To maintain the required matching of the air chamber pressure to the fluid (typically liquid or fibrous slurry) pressure, and hence keep the diaphragm approximately in the middle of its range of movement, regardless of drifting of the running-average fluid pressure, the two-way port valve regulated the outflow of air from the chamber by bleeding the air therethrough from the chamber. Thus, air was exhausted from the air tank by the diaphragm being momentarily lowered away from the end of the bleed valve, thereby exposing an opening, or openings, in the end of the bleed valve head through which the pressurized air rushed to be discharged to the atmosphere via a connecting pipe. When the air pressure within the attenuator tank balanced, or became slightly less than, the normally slowly drifting, running-average fluid pressure within the attenuator's fluid conduit on which the attenuator's air chamber was mounted, the diaphragm remained or was urged against the hole, or holes, in the valve head to close off air discharging from the air tank through the hole, or holes, to the atmosphere via the connecting, isolated pipe. This temporary blocking of the air-escape route, combined with the continuous inflow of air via the air-input path, permitted the air pressure to build up in the tank again until a slight excess of air pressure overcame the slight suction force arising from the pressure differential across the thickness of the diaphragm at the bleed valve's hole, or holes, and again displaced the diaphragm away from the bleed valve, thus again allowing air to escape through the now exposed hole, or holes, in the valve head. It is important to note that the valve operated to exhaust the air from the attenuator air chamber, but not to introduce air into the tank.
In this first type of two-port bleed-only valve, the valve head protruded below the grid-like portion of the (ceiling-plus-floor) "wall" between air chamber and fluid-flow conduit at a fixed distance. This protrusion of the valve head yielded an average position of the diaphragm which was far enough below the rigid back-up grid that the diaphragm could move freely, within limits, in either upward or downward directions to accommodate either positive or negative pulses with respect to the average pressure and flow.
In the second type of prior attenuator valve configuration, the valve head also contained ports therein which, when not blocked by the diaphragm, permitted air from the attenuator's pressurized air tank to pass over and into the head of the valve, and then, via the output pipe portion of the bleed system, out of the attenuator air chamber to the atmosphere. However, in this other prior configuration, the valve head did not have a fixed protrusion, but instead could retract a short distance, typically one inch, upon arrival of the stock in the attenuator, and remain retracted, nearly flush with the surface of the back-up grid, until the gradually rising air pressure nearly matched the nearly-constant fluid pressure. At that time, the rising air pressure would displace the diaphragm downward away from the back-up grid, and the valve head would follow the diaphragm downward to arrive and remain at its steady-state operating position, furnishing a pre-selected protrusion, typically one inch. Specially designed movability of the valve head allowed the valve to accommodate the initial temporary upward displacement of the diaphragm during the interval of significant mismatch of pressures without producing an undue stress, strain or deformation in it which would have arisen from the localized interference from a fixed-protrusion valve head. With this significant refinement, this type of valve also operated as a bleed valve only, regulating only the escape of air from the attenuator air chamber.
One problem with the first of the prior types of valve, and to a lesser degree with the second type which had modified, multiple holes, arises from the fact that the valve is essentially either open or closed. Further, there is a hysteresis effect since some force is required to break the diaphragm away from the suction at the holes. An example of a typical prior valve head of the first type, showing the holes therein, is illustrated in FIG. 12 of U.S. Pat. No. 4,030,971.
The movement of the diaphragm tends to be rather abrupt, particularly when the valve hole is being opened. These sudden movements of the diaphragm, not related to absorbing any sharp pulse in the fluid, introduce their own pulses (spurious "noise") into the fluid, thus degrading the performance of the attenuator as a reducer of unwanted fluid noise (pulsations).
A second, related problem arises from the fact that both types of valve are used in conjunction with an essentially-constant inflow of air, via a separate channel, to the attenuator air chamber, and this involves a compromise. To pressurize the air chamber, at each start-up, in a reasonable interval of time, the inflow of air should be at a relatively high rate. To avoid the complications of additional control components, the relatively high rate of inflow is retained during subsequent operation of the attenuator. This, in turn, requires the bleed valve to dispose of a relatively large amount of air in a given time, unfortunately increasing the noise generated in the fluid by the suddenness of the operation of the air bleed.